The present invention relates generally to phosphoric acid esters, and more particularly to a continuous process for producing monomeric bisaryl diphosphates.
Bisaryl diphosphates, such as bisphenol A bis(diphenyl)-phosphate are known to be effective flame retardants for polymer resins. For example, a variety of polyphenylene oxide/high-impact polystyrene (xe2x80x9cPPO/HIPSxe2x80x9d) and polycarbonate/acrylonitrile-butadiene-styrene (xe2x80x9cPC/ABSxe2x80x9d) blends all can be improved with bisaryl diphosphate flame retardants.
When using bisaryl diphosphates to impart flame retardancy to plastics it is desired to use compounds having a high percentage of the monomer. This is because monomeric bisaryl diphosphates impart beneficial physical properties to the polymer, which properties are not provided by their dimeric or polymeric counterparts. For example, resins to which monomeric bisaryl diphosphates have been added exhibit improved impact strength, melt flow index, tensile properties and flexural properties when compared to resins combined with dimeric or polymeric aryl phosphates.
Because of their commercial utility, various processes for the manufacture of monomeric bisaryl diphosphates have been developed. For example, it is known that bisphenol A bis(diphenyl)-phosphate can be obtained by catalytically reacting a phosphorus oxyhalide with bisphenol A (BPA) and then reacting the intermediate with phenol.
Prior art processes for making bisphenol A bis(diphenyl) phosphate have been batch processes; neither the reaction of the phosphorus oxyhalide with bisphenol A, nor the reaction of the intermediate with phenol, has been done continuously. However, while batch processes can be adjusted to produce a large amount of monomeric bisaryl diphosphate product, the productivity of such reactions is generally not satisfactory.
Continuous processes may be employed to obtain higher productivity for certain synthetic pathways compared to batch processes. However, application of a continuous process in the formation of phosphoric acid esters would be expected to produce small amounts of monomeric product compared to dimeric, other oligomeric, or polymeric product. Accordingly, the available synthetic pathways suffer from inefficiencies that make them unsuitable for large scale commercial application.
In view of the above, it can be seen that a need exists for improved methods of making phosphoric acid esters such as bisphenol A bis(diphenyl) phosphate. The present invention addresses that need.
Briefly describing one aspect of the present invention, there is provided a method of making phosphoric acid esters. The method preferably comprises:
(1) continuously reacting a phosphorus oxyhalide with a polyol to produce at least about 60% normalized monomeric halophosphate intermediate; and
(2) reacting the monomeric halophosphate intermediate with an alcohol to produce a desired phosphoric acid ester.
In some preferred embodiments, the polyol in Step 1 is a dihydric alcohol and the alcohol in Step 2 is a monohydric alcohol. Conversely, in some preferred embodiments the phosphorus oxyhalide is reacted with a monohydric alcohol to produce at least about 60% normalized monohalomonophosphate diester intermediate. In some embodiments, both the Step 1 and Step 2 reactions are carried out continuously.
One object of the present invention is to provide a method for continuously producing phosphoric acid esters.
Another object of the present invention is to provide a method for producing phosphoric acid esters in a continuous reaction wherein the normalized monomeric halophosphate intermediate content of a reaction between phosphorus oxyhalide and a polyol is at least about 60%, preferably at least about 70%, more preferably at least about 80% and most preferably at least about 90%.
Another object of the present invention is to provide a method for producing phosphoric acid esters in a continuous reaction wherein the normalized monohalomonophosphate diester content of a reaction between phosphorus oxyhalide and a monohydric alcohol is at least about 60%, preferably at least about 70%, more preferably at least about 80% and most preferably at least about 90%.
A further object of the invention is to provide a method for producing monomeric phosphoric acid ester products which can be used as flame retardants, for example, in plastics.
Further objects and advantages of the present invention will be apparent from the following description.